The Britain-based Syrian Observatory for Human Rights says at least 14 children have been killed in air raids since Monday evening.

A girl who fled Raqqa city fills water containers at a camp north of Raqqa. August 7, 2017.

US-led coalition air strikes against Daesh in northern Syria have killed 29 civilians over the past 24 hours, a war monitor alleged on Tuesday.

The Britain-based Syrian Observatory for Human Rights (SOHR) said that nine women and 14 children died in coalition air raids on Raqqa city since Monday evening. It said 14 of the dead were members of one family, adding the toll could rise because of the number of critically wounded.

The US-led coalition battling Daesh in Syria and Iraq entered its fourth year of operations on Tuesday. It began on August 8, 2014 in Iraq, moving later to battle Daesh in neighbouring Syria.

Smoke rises from Raqqa, Syria on July 28, 2017. US and Russian-backed forces have killed at least 19,663 civilians in Syria and Iraq between August 2014 and July 2017, monitoring group Airwars estimates. (Reuters Archives)

"Staggering loss of civilian life"

The Observatory, which relies on a network of sources inside Syria for its information, has regularly reported civilians killed by coalition strikes.

The US has admitted that its strikes caused the deaths of 624 civilians since 2014. Rights organisations say the number is far higher.

US and Russian-backed forces have killed at least 19,663 non-combatant civilians in Syria and Iraq between August 2014 and July 2017, monitoring group Airwars estimates.

United Nations war crimes investigators in June expressed alarm at the rise in civilian deaths in the fight for Raqqa.

Intensified air strikes have caused "staggering loss of civilian life" and led to 160,000 civilians fleeing their homes, Paulo Sergio Pinheiro, head of the UN Commission of Inquiry, said.

Captured by Daesh in 2014, Raqqa became a symbol of atrocities by its fighters in its self-declared "caliphate."

The US-backed alliance of Syrian groups began a campaign to capture the city last year, slowly encircling the city before breaking into it for the first time in June. They have since advanced and taken over around 45 percent of the city.

Daesh has responded with a barrage of car bombs, suicide bombers, weaponised drones and snipers.

To the southeast, Syrian government forces backed by their Russian allies are fighting Daesh in the oil-rich province of Deir Ezzor.

More than 400,000 people have been killed in Syria since the war began with anti-regime protests in March 2011.